[TASK] Introduce $GLOBALS['TYPO3_CONF_VARS']['DB']['port']

The [db][host] setting can contain a hostname, followed by a colon and
a port. This was ok with mysql, as it accepted host and port in one
parameter.
With the switch to mysqli, host and port must be given as single
parameters, forcing the core to split host and port. This can fail
especially if the host is an ipv6 address.
In order to separate this in a better way, the new "port" parameter
in TYPO3_CONF_VARS DB is introduced. If set, the host will not be
splitted by colon anymore, but host and port setting will be used
as given.

Resolves: #47455
Releases: 6.2, 6.1
Change-Id: I3096d166e574d9b6045ac92ed3dc8b2c48ef94d2
Reviewed-on: https://review.typo3.org/20321
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn